This invention relates to optical scanning devices such as bar code scanners, and more particularly to beam-shaping systems for generating a scanning laser beam adapted for use with a selected broad range of working distances and symbol densities.
Bar code readers are known in the prior art for reading various symbologies such as UPC bar code symbols appearing on a label or on the surfaces of an article. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers in scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into information, typically descriptive of the article or some characteristic thereof. Such information is conventionally represented in digital form and used as an input to a data processing system for applications in point-of-sale processing, inventory control and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. No. 5,600,121, assigned to the same assignee as the instant application. Such systems may employ a portable laser scanning device held by a user, which is configured to allow the user to aim the device, and more particularly, a scanning laser light beam, at a targeted symbol to be read.
The light source in a laser scanner bar code reader is typically a semiconductor laser. The use of semiconductor devices as the light source is especially desirable because of their small size, low cost and low voltage requirements. The laser beam is optically modified typically by an optical assembly, to form a beam spot of a certain size at the target distance. It is preferred that the cross section of the beam spot at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
In the laser beam scanning systems known in the art, the laser light beam is directed by a lens or other optical components along the light path toward a target that includes a bar code symbol on the surface. The moving-beam scanner operates by repetitively scanning the light beam in a line, pattern or series of lines across the symbol by means of motion of a scanning component, such as a moving mirror placed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line across the pattern of the symbol, or scan the field of view of the scanner, or both.
Bar code reading systems also include a sensor or photo detector which detects light reflected or scattered from the symbol. The photo detector or sensor is positioned in the scanner in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. The light is detected and converted into an electrical signal.
Some bar code reading systems are xe2x80x9cretro-reflectivexe2x80x9d. In a retro-reflective system, a moving optical element such as a mirror is used to transmit the outgoing beam and receive reflected light. Non-retro-reflective systems typically employ a moving mirror to transmit the outgoing beam and a separate detection system with a wide, static field of view.
Electronic circuitry and software decode the electrical signal in a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photo detector is converted by a digitizer into a pulse or modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded, based on the specific symbology used by the symbol, into a binary representation of the data encoded in the symbol, and subsequently to the information or alphanumeric characters so represented. Such signal processors are disclosed in U.S. Pat. No. 5,734,152 assigned to Symbol Technologies, Inc. which patent is hereby incorporated by reference.
Different bar codes have different information densities and contain a different number of elements in a given area representing different amounts of encoded data. The denser the code, the smaller the elements and spacings. Printing of the denser symbols on an appropriate medium is exacting and thus is more expensive than printing symbols with larger elements. The density of a bar code symbol can be expressed in terms of the minimum bar/space width, also called xe2x80x9cmodule sizexe2x80x9d, or as a xe2x80x9cspatial frequencyxe2x80x9d of the code, which is in the inverse of twice the bar/space width.
A bar code reader typically will have a specified resolution, often expressed by the module size that is detectable by its effective sensing spot. For optical scanners, for example, the beam spot size may be somewhat larger than the minimum width between regions of different light reflectivities, i.e., the bars and spaces of the symbol. The resolution of the reader is established by parameters of the beam source or the detector, by lenses or apertures associated with either the beam source or the detector, by the angle of beam inclination with respect to the plane of the symbol, by the threshold level of the digitizer, by the programming in the decoder, or by a combination of two or more of these elements. The photo detector will effectively average the light scattered from the area of the projected spot which reaches the detector aperture.
The region within which the bar code scanner is able to decode a bar code is called the effective working range of the scanner. Within this range, the spot size is such as to produce accurate readings of bar codes for a given bar code density. The working range is dependent on the focal characteristics of the scanner components and on the module size of the bar code.
Many known scanning systems collimate or focus the laser beam using a lens system to create a beam spot of a given diameter at a prescribed distance. The intensity of the laser beam at this point, in a plane normal to the beam (ideally approximately parallel to the scanned symbol), is ordinarily characterized by a xe2x80x9cGaussianxe2x80x9d distribution with a high central peak. Gaussian beams typically have a profile along their axis of propagation exhibiting a waist (collimated) zone with limited divergence followed by a divergence zone thereafter. The collimated zone determines a depth of field (focusing range) for maximum bar code density. The working range is defined as the region within which the scanned beam spot is sufficiently well formed that its detected scattered radiation can be decoded by the scanner. But as the distance between the scanner and the symbol moves out of the working range of the scanner, which is typically only a few inches in length, the Gaussian distribution of the beam spot greatly widens, preventing accurate reading of a bar code. Such scanning systems, accordingly, must be positioned within a relatively narrow range of distances from a symbol in order to properly read the symbol.
It has been proposed to create a laser scan beam by directing a collimated beam of laser light onto a linear axicon optical element, for example, a conical lens, to produce a beam of light which exhibits a consistent spot size over a substantial distance along the axis of the beam. Such an optical system is disclosed in U.S. Pat. No. 5,331,143 to Marom et al. and assigned to Symbol Technologies, Inc., which patent is hereby incorporated by reference.
The aforementioned axicon system produces a nearly diffraction free beam. The use of such a beam has been proposed to maximize the focusing limited working range of the scanning beam. Such a beam exhibits substantially no divergence over a relatively long distance range and then breaks into a donut like spot pattern of intensity distribution. Such a non-diverging beam can provide two to three times the range of a conventional Gaussian beam for a particular bar-code density. However, as noted by applicants, where such a beam is designed to improve performance in scanning a certain bar code density, the corresponding working ranges of lower density symbols are not increased significantly or at all, being limited by the distance where the beam breaks into a donut-like distribution (xe2x80x9cdonut distancexe2x80x9d).
Accordingly, it is a primary object of the present invention to optimize and maximize the working ranges of a bar code scanner for a wide variety of bar code densities.
Some conventional scanners employ two laser optical systems: one a short working range system and the second a long working range system. Such scanners have multiple operating modes, which allow for the selection of a different optical system, spot sizes and scanning beam depending on the distance of the target system. However, such systems are larger and more complicated than single source scanners, and consequently tend to be more expensive and less desirable for all these reasons.
It is another object of the present invention to provide a compact and inexpensive bar code scanner with improved working ranges and variety in the bar code densities which can be read at such ranges.
It is known that in certain scanning applications, an elongated or elliptical spot may be used to improve the performance of the scanning system. See, for example, U.S. Pat. No. 5,648,649 to Bridgelall. When an elliptical spot is employed, ideally the major axis of the spot is oriented parallel to the major axes of the bars making up the target code and perpendicular to the direction of beam scan. Such a spot form may reduce inaccuracies in reading the code, because, e.g. an elliptical spot system is somewhat less susceptible to errors introduced by voids and spreads in the symbol and speckle noise.
Conventionally, the elliptical spot is generated from a Gaussian distribution laser beam by introducing a cylindrical optical power in a direction perpendicular to the scanning direction and/or employing an elliptical or rectangular exit pupil. Applicants have observed that such techniques are ineffective in producing an effective spot geometry when an axicon laser beam is employed.
Accordingly, it is another object of the present invention to provide improved techniques for producing an elongated spot geometry in a laser scanning system employing both Gaussian and non-Gaussian laser beams.
In a bar code scanner which relies on a precise scanning laser beam profile and spot geometry, changes in temperature that affect the optical properties of the beam producing system can degrade the performance of the system such as by reducing effective working range, reducing readable code density at a particular distance and generally reducing the accuracy of the system. Various techniques have been proposed for reducing temperature variation (xe2x80x9cathermalizingxe2x80x9d) optical systems in bar code readers. Such systems are disclosed, for example, in U.S. Pat. No. 5,673,136 to Inoue and U.S. patent application Ser. No. 09/109,018 filed Jul. 1, 1998 to Li et al. and assigned to Symbol Technologies, Inc., which application is hereby incorporated by reference.
Typically glass optical elements are less affected by temperature changes than are plastic optical elements having the same nominal optical properties. Typically, also, plastic optical elements are less expensive and less difficult to fabricate and replicate.
It is a further object of the present invention to provide improved athermalized optical systems for laser scanners, both for conventional laser scanners and scanners adapted to achieve the other objects of the present invention.
These and other objects and features of the present invention will be apparent from this written description and the associated drawings.
The present invention relates to an improved axicon optical system for scanning optical codes. The improved beam-shaping systems of the present invention are capable of generating nearly diffraction-free beams to improve the ability to scan bar codes. A laser beam produced thereby has a central peak having a controlled divergence and may be used to produce an elongated scanning spot. Aspects of the present invention also relate to a laser assembly which may be used to produce scanning beams including the above-mentioned diverging beam with elongated spot. The apparatus combines various optical functions in structures which are relatively insensitive to temperature change.
The present invention includes an optical code scanner employing a diverging, nearly diffraction free laser beam to scan optical code symbols. The source of the laser beam may include a laser diode and an optical system positioned with respect to the laser diode and configured to produce a diverging laser beam with a generally conical wave front which flattens radially outwardly from an axis of propagation of the beam. In preferred embodiments, the nearly diffraction-free beam has an elongated transverse intensity distribution which forms an enlongated spot on a reference surface perpendicular to an axis of propagation of the beam at a minimum working distance from the scanner. The central peak in the transverse intensity does not split into rings or donuts within the designed working range. For example, the beam produces a spot at the minimum working distance of the scanner which has a dimension d0 in the direction of scanning which is less than 13 mils and a dimension d, greater than 160 mils at the maximum working distance of the scanner. For example, embodiments of the scanner are capable of using the laser beam to read 7.5 mil code at a minimum working distance of less than 9 inches and to read 100 mil code at a maximum working distance greater than 520 inches.
The present invention also includes a laser beam for use in scanning bar codes. The inventive beam has a wave front which deviates from a reference plane perpendicular to its axis of propagation, the deviation being characterized by a phase whose dependence is given by
W(r)=xcex1rxe2x88x92xcex2r2
where W(r) is the deviation; where r is radial distance from the axis of propagation; where xcex1 is selected to produce a spot at the minimum scanning distance with a diameter d0 sufficient to permit the reading of the highest density bar code to be scanned; and where xcex2 is selected to provide optimum working ranges for bar codes of different densities. In preferred embodiments, a is selected between 0.2xc3x9710xe2x88x923 and 6xc3x9710xe2x88x923. xcex2 is selected so that       λ          10      ·              R        0        2               less than   β   less than             2      ·      α              R      0      
where xcex is the laser wavelength, where R0 is the starting radius of the laser beam at the system aperture. More preferably,       α          5      ·              R        0               less than   β   less than       α          R      0      
The present invention also teaches the construction of an optical system having at least one optical element for partially collimating the laser beam, providing optical power to reduce beam divergence and for producing a wave front as described in Equation 24 below. In preferred embodiments, the optical system includes a plano convex lens for partially collimating the laser beam and an optical element having a substantially flat first surface perpendicular to the optical axis of the system and a second surface defined by a figure of rotation formed by rotating a line about the optical axis at an acute angle to define an optical element which causes a phase tilt in the beam inward toward the optical axis. The beam so produced has a transverse field distribution that can be expressed as a series containing the Bessel functions. Essentially only the axial peak of beam is employed for scanning.
The present invention also includes a method of producing a laser beam for scanning an optical code. A diverging laser beam is provided by a semiconductor laser. The divergence of the laser beam is reduced by, for example, a lens to a predetermined non-zero divergence. A generally conical wave front deformation is imposed on the laser beam, for example by a lens with a conical surface. These steps can be performed in any order or simultaneously. Such a diverging axicon laser beam is produced for scanning optical code. The beam may be used in a method of scanning a symbol. According to this method, a beam of laser light is generated. The transverse intensity pattern of the beam shows a central peak surrounded by a plurality of side lobes symmetrically located on either side of the central peak. The beam has a high degree of energy confinement near the propagation axis when the beam travels to the symbol. The modified beam may be moved across the symbol to scan the symbol.
Another aspect of the present invention relates to a laser scanner apparatus for producing a laser beam with an elongated spot for scanning a symbol. The apparatus includes the use of a partially reflective mirror, which is placed in the optical path of a circular or non-circular diffraction-free beam. Light is partially transmitted and partially reflected. However, the reflected portion of the beam returns to the same direction as the transmitted beam by a second reflection on the back of the mirror. A nearly diffraction-free beam is obtained with an elongated transverse intensity distribution by a superposition of the two portions of the incident beam. Advantageously, the two portions are sufficiently displaced so that optical interference of these two portions is negligible. Centers of the beam portions may be displaced from one another along an axis perpendicular to a direction of scanning of the laser scanner apparatus. The result is an elongated region of illumination on a reference plane perpendicular to the beam portions in at least part of a working range of the laser scanner apparatus. Advantageously, the beam portions are sufficiently displaced so that they have minimum overlap. Preferably, the optical system used to produce the elongation is a single period diffraction grating with a cosine-shaped profile.
In an alternative embodiment, the optical system includes an optical plate having two planar refractive surfaces which are non-parallel and from each of which a portion of the laser beam is propagated. In yet another alternative embodiment, the optical system includes a partially reflective plate having two planar reflective surfaces from each of which a portion of the laser beam is propagated.
The present invention also contains techniques for enhancing the temperature stability of the laser beam source. A lens system may be employed having a glass lens with at least one spherical surface; and a molded plastic element having a non-planar refractive surface located on an optical axis of the glass lens. The combination of the plastic lens and the glass lens provides an aspherical lens system with essentially all of the optical power in the glass lens and with greater temperature stability than an optically equivalent aspherical plastic lens. The plastic element may include at least one surface configured to produce the generally conical wave front deformations described herein. The molded plastic element may also provide a single period grating or provide an aberration correction for the glass lens. Preferably, rotationally symmetric elements can be formed on one surface of the plastic element, and cylindrical elements can be formed on the other surface.
An athermalized laser assembly may be constructed from a laser diode, a lens system, and first and second members for maintaining the relative positions of the laser diode and lens system. The first member may be a tube having a first coefficient of thermal expansion and effective length L in the direction of an optical axis of the laser assembly. The second member may be a cylindrical sleeve. The sleeve may have a second, larger coefficient of thermal expansion and an effective length Lp in the direction of the optical axis of the laser assembly. The sleeve and tube may be telescoping and together confine the separation between the laser diode and lens system to a desired spacing along the optical axis of the assembly. The lengths L and Lp and the thermal properties of the tube and sleeve are selected so that linear expansion of the sleeve in one direction along the optical axis of the assembly substantially cancels out the linear expansion of the tube in the opposite direction.
The laser assembly may be constructed with a lens system which includes a lens with at least one spherical surface. A lens element integral with the sleeve provides a conical surface and a single period diffraction grating with a cosine-shaped profile. The laser diode and lens system may be positioned with respect to one another and configured to produce a diverging, nearly diffraction free beam having an elongated region of illumination in a reference plane perpendicular to the laser beam within a working range of the laser assembly.
The foregoing is intended to summarize certain aspects of the invention. The subject matter intended to be protected is, however, defined by the claims and equivalents thereof.